Asynchronous electric motors, which are regarded as particularly robust, and can be obtained with high power ratings, are commonly used for driving heavy machinery, e.g. scraper chain conveyors, coal ploughs and shearer loaders used in mines. Especially where several motors are harnessed together to drive one machine, it is necessary to determine the power output of the individual motors, in order that the various drives can be harmonized so that the load handled by the driven machine is evenly taken up by the various drive motors. Since the loading of the machines, and hence of the motors which drive them, is continually changing, rapid determination of the power output of the individual motors, giving the possibility of rapid adjustment, is desirable.
Hitherto the usual way of determining the power of asynchronous motors has been to measure the voltage supplied to them, and the active current. The active power is then obtained in a known manner by multiplying the voltage by the active current. For many power supply systems, especially underground systems, this known method is difficult and imprecise. It involves taking readings directly at the current supply to the motors, which is very costly, particularly where a large number of motors have to be monitored.